1. Field of the Invention
Embodiments of the invention generally relate to enhanced oil recovery methods. More specifically, embodiments of the invention relate to methods of recovering oil from a reservoir using a downhole steam generation drive process after a cold heavy oil production with sand process.
2. Description of the Related Art
Oil can generally be separated into classes or grades according to its viscosity and density. Grades of oil that have a high viscosity and density may be more difficult to produce from a reservoir to the surface. In particular, extra heavy oil requires enhanced oil recovery techniques for production. In the following description, the generic term “oil” includes hydrocarbons, such as extra heavy oil, as well as less viscous grades of oil.
A large portion of the world's potential oil reserves is in the form of heavy or extra heavy oil, such as the Orinoco Belt in Venezuela, the oil sands in Canada, and the Ugnu Reservoir in Northern Alaska. Currently, some existing oil reservoirs are exploited using thermal enhanced oil recovery techniques that usually result in recovery efficiencies within a range of about 20% to 75%. One of the most common thermal enhanced oil recovery techniques is surface steam injection by which heat enthalpy from the steam is transferred to the oil by condensation. The heating reduces the viscosity of the oil to allow drainage and collection. Thus, oil recovery is high if the temperature can be maintained near the temperature of the surface injected steam.
In the Arctic, however, below the surface and extending to depths of 1500 feet or more, permafrost layers exist. It is thus impractical to generate steam on the surface and inject it into the formation below because the steam would have to pass through the permafrost layer. The high temperature steam may melt the permafrost layer, thereby causing it to expand and potentially crush any wellbores extending through the permafrost layers into the oil reservoirs below.
Alternatively in deep reservoirs or thin reservoirs, much heat is lost through the wellbore to the rock surrounding the reservoir. Then traditional steam injection is little more than a hot water flood and loses much of its effectiveness in reducing the oil's viscosity and improving oil production.
A current practice is to use Cold Heavy Oil Production with Sand (“CHOPS”). As the name implies, this utilizes primary production without heat. In general, a well is drilled into an unconsolidated reservoir, such as a highly porous tar sand formation. The well is perforated and a pumping device may be lowered into the well. The combination of reservoir pressure and artificial lift provided by the pumping device drives the oil in the reservoir to the well surface. Sand influx with the oil is encouraged by increasing the “draw down” pressure in the well (i.e. the differential pressure that drives fluids from the reservoir into the well), which enlarges the access of oil flow and decreases the resistance of fluid flow. A mixture of heavy oil and sand is produced and separated at the surface. One shortcoming of CHOPS is that the recovery efficiency can be as low as 5 percent of the original oil in place. Another shortcoming is that after the economic production limit is reached using the CHOPS process, the reservoir may not be suitable for other enhanced oil recovery techniques.
As the number of potential heavy oil reservoirs increases and the complexity of the operating conditions of these reservoirs increases, there is a continuous need for efficient enhanced oil recovery techniques and methods.